Stability order (opposite to carbocations): CH3- > 1 degree > 2 degree > 3 degree (for simple alkyl carbanions)
Stabilizing factors:
- -I effect: More electronegative groups stabilize negative charge
- Resonance: Allylic, benzylic carbanions are stabilized
- Aromaticity: Cyclopentadienyl anion (C5H5-) has 6 pi electrons — aromatic and very stable
- s-character: sp > sp2 > sp3 (higher s-character holds electrons closer to nucleus)
Acidity connection: The more stable the carbanion (conjugate base), the more acidic the parent compound. Acidity: RC≡CH > C6H6 > CH2=CH2 > CH3CH3 (sp > sp2 > sp3)